In St Ives, a constituency won by the Conservatives from the Lib Dems in 2015, some 38.3 per cent are self-employed.

This is the highest level of self-employment for any constituency in Britain, according to the ONS.

Losing 11 seats in Parliament would leave Theresa May with a remaining 319 Conservative MPs - six short of an absolute majority of 325. David Cameron managed to secure 331 in the 2015 General Election.

Of the 11 seats at risk, five had the Lib Dems in second place in 2015, while Labour was in second in four of them and both the SNP and UKIP were second in one.

In total there are 101 constituencies where more than one fifth of the working population are self-employed. The Conservatives won 67 of these seats at the last General Election.

They are predominately located in the south of England with self-employment rates in Cornwall and Sussex - two counties which are traditionally bastions of Conservative support - being particularly high.

The Prime Minister's spokesperson has refused four times to refuse to rule out a review of the Government's controversial plans to hike national insurance, which would affect some self-employed people in the Class 4 NIC category.